Method for the liquid phase epitaxial growth of semiconductor crystals

ABSTRACT

A method for the liquid phase epitaxial growth of semiconductor crystals is performed using apparatus that comprises a dis-shaped vessel provided with a plurality of receptacles of a source solution for said epitaxial growth; a similarly disc-shaped boat disposed close to the underside of the vessel and provided with a recess for receiving a substrate on which there are to be formed semiconductor crystals by liquid phase epitaxial growth, said recess being cut out in that side of the boat which faces the vessel so as to be brought exactly under any of the receptacles of the vessel when it is rotated; a quartz pipe for supporting the boat; means for rotating either of the vessel and boat relative to the other; an envelope surrounding the vessel and boat in airtight relationship; means for conducting a desired type of gas through the envelope; and a cylindrical furnace enclosing the envelope to heat its interior with the relative position of said cylindrical furnace and envelope rendered adjustable.

United States Patent [191 Unno et a1.

[ METHOD FOR THE LIQUID PHASE EPITAXIAL GROWTH OF SEMICONDUCTOR CRYSTALS [75] Inventors: Yoichi Unno, Kawasaki; Motoyuki Yamamoto, Kamakura; Haruki Kurihara, Tokyo, all of Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan 22 Filed: July 24,1972

21 Appl. No.: 274,601

[30] Foreign Application Priority Data July 30, 1971 Japan 46/67333 [52] U.S. Cl 148/171, 148/172, 117/201, 118/415, 118/412 [51] Int. Cl. H011 7/38 [58] Field of Search 148/171, 172; 117/201; 118/415, 412, 426

[56] References Cited UNITED STATES PATENTS 3,631,836 1/1972 Jarvela et a1 118/415 3,565,702 2/1971 Nelson 148/172 Oct. 16, 1973 3,578,513 5/1971 Pilkuhn et al 148/171 Primary Examiner-G. T. Ozaki AttorneySolon B. Kemon et a1.

[ 5 7] ABSTRACT A method for the liquid phase epitaxial growth of semiconductor crystals is performed using apparatus that comprises a dis-shaped vessel provided with a plurality of receptacles of a source solution for said epitaxial growth; a similarly disc-shaped boat disposed close to the underside of the vessel and provided with a recess for receiving a substrate on which there are to be formed semiconductor crystals by liquid phase epitaxial growth, said recess being cut out in that side of the boat which faces the vessel so as to be brought exactly under any of the receptacles of the vessel when it is rotated; a quartz pipe for supporting the boat; means for rotating either of the vessel and boat relative to the other; an envelope surrounding the vessel and boat in airtight relationship; means for conducting a desired type of gas through the envelope; and a cylindrical furnace enclosing the envelope to heat its interior with the relative position of said cylindrical furnace and envelope rendered adjustable.

7 Claims, 11 Drawing Figures PATENTEDHBI 16 ms 3.765959 SHEEI 1 0F 2 FIG. 2

PATENTEDHCT 1 5m 3,765959 sum ear '2 s FIG.7

TEMPERATURE TIME/(SHOND) METHOD FOR THE LIQUID PHASE EPITAXIAL GROWTH OF SEMICONDUCTOR CRYSTALS BACKGROUND OF THE INVENTION This invention relates to a method for the liquid phase epitaxial growth of semiconductor crystals and more particularly to a type adapted to attain the liquid phase epitaxial growth of said crystals in multiple layers.

Where there are formed heterojunctions such as those consisting of GaAs-Ga Al As by the liquid phase epitaxial growth method (hereinafter abbreviated as the LPE method"), there has heretofore been used a horizontal heating furnace containing a plurality of bath tanks. In this case, there is required a boat elongated in the axial direction of said furnace. Use of a lengthy boat makes it necessary to' extend that portion of the furnace where uniform temperature should be maintained in order to equalize the temperature of each bath tank arranged in the direction in which the boat is made to travel. However, manufacture of a furnace where uniform temperature should be maintained over a considerable portion is accompanied with great difficulties. Particularly where semiconductor crystals are to be created in more than four layers by the LPE method on a large substrate, then there would have to be used a furnace where large uniform temperature should be maintained over an extremely extended region. Manufacture of such a furnace is practically next to impossible.

SUMMARY OF THE INVENTION It is accordingly the object of this invention to provide a method for the liquid phase epitaxial growth of semiconductor crystals which enables the liquid phase epitaxial growth of said crystals to be easily effected in several layers even on a large substrate, and is moreover capable of readily controlling the temperature distribution of a source solution for said epitaxial growth and that of the substrate.

According to an aspect of this invention, there is provided an apparatus for the liquid phase epitaxial growth of semiconductor crystals which comprises a discshaped vessel provided with a plurality of receptacles equidistantly arranged from the center so as to be filled with a source solution for said epitaxial growth; a similarly disc-shaped boat disposed close to the underside of the vessel and provided with a recess for receiving a substrate on which semiconductor crystals are to be grown, said recess being cut out in that side of the boat which faces the vessel so as to be brought exactly under any of the recesses of the vessel when it is rotated; a cylindrical support for holding the boat; means extending through the interior of the cylindrical support so as to rotate either of the vessel and boat relative to the other; an envelope surrounding the vessel and boat in airtight relationship; means for conducting a desired type of gas through the interior of the envelope; a cylindrical furnace enclosing the envelope to heat its interior; and means for adjusting the relative position of the envelope and cylindrical furnace.

BRIEF DESCRIPTION OF THE DRAWINGS An apparatus for the liquid phase epitaxial growth of semiconductor crystals embodying this invention is presented in the appended drawings, in which:

FIG. 1 is a schematic perspective view, partly broken away, of said apparatus;

FIG. 2 is a sectional view of the main part of the apparatus of FIG. 1;

FIG. 3 is a perspective view of a disc-shaped vessel of a source solution for the liquid phase epitaxial growth of semiconductor crystals and a similarly discshaped boat;

FIG. 4A is a plan view of a rotation drive shaft and FIG. 4B is a longitudinal sectional view thereof;

FIG. 5A is a top view of the disc-shaped boat and FIG. 5B is a sectional view on line XX of FIG. 5A;

FIG. 6A is a bottom view of a disc-shaped vessel of a source solution for the liquid phase epitaxial growth of semiconductor crystals; and FIG. 6B is a sectional view on line YY of FIG. 6A;

FIG. 7 is a curve diagram indicating a distribution of temperature in the furnace; and

FIG. 8 shows a gradient of temperature in the source solution for the liquid phase epitaxial growth of semiconductor crystals when they are created.

DETAILED DESCRIPTION OF THE INVENTION There will now be described by reference to the appended drawings an apparatus for the liquid phase epitaxial growth of semiconductor crystals according to an embodiment of this invention.

Referring to FIG. 1, number 1 is a cylindrical furnace having the inside of the wall 2 mainly formed of heat insulating material wound with a heating wire 3. This heating wire 3 is intended to heat the body of the apparatus received in the cylindrical furnace 1 for the liquid phase epitaxial growth of semiconductor cyrstals. Number 4 represents an envelope surrounding said apparatus which is formed of, for example, a quartz pipe mm in inner diameter, 450 mm high and closed at one end, with the open end fixed airtight to a base 5 with a polyfloroethylene gasket. In the envelope 4 are received a disc-shaped vessel of a source solution for the liquid phase epitaxial growth of semiconductor crystals, a similarly disc-shaped boat 7 and a cylindrical support 8 for holding said vessel and boat. Into the cylindrical support 8 are inserted a rotation drive shaft 9 and a pipe containing thermocouple 21.

The vessel 6 is shaped like a disc 40 mm in diameter, 10 mm high and bored at the center with a square aperture 6a, each side measuring 8 mm (FIG. 6A). The boat 7 is similarly shaped like a disc 40 mm in diameter, 15 mm high and perforated at the center with a circular hole 7a 6.5 mm in diameter (FIG. 5A). The support 8 also takes a cylindrical form 370mm high, with the upper end face bored at the center with a circular aperture 8a 6.5 mm in diameter. The vessel 6 and boat 7 are made of, for example, high purity graphite and the sup port 8 is prepared from quartz. All these members are disposed on one another in the order mentioned. The central apertures of said members 6, 7 and 8 are penetrated by the rotation drive shaft 9, the lower end of which passes through the support 8 to be connected to a source of rotation drive (not shown) positioned below the subject apparatus, thereby rotating, as described below, the vessel 6 alone with the boat 7 kept at rest. The rotation drive shaft 9 in FIGS. 4A and 48 has a stop portion 10 formed at the upper end and is shaped like a square columnar form 11 below said stop 10. This square columnar portion 11 fits into the square aperture of the disc shaped vessel 6. Further, said shaft 9 takes a round columnar shape 12 below said square columnar portion 11. This round columnar portion tits into the round holes 7a and 8a of the boat 7 and support 8. Accordingly, when the drive shaft 9 rotates, the vessel 6 turns at the same time, whereas the boat 7 and support 8 are not rotated, but kept at rest.

The boat 7 is further provided, as shown in FIGS. A and SB, with a recess 13 for receiving a substrate on which crystals are to be grown, said recess 13 being cut out in that side of the boat 7 which faces the underside of the vessel 6.

The recess 13 is preferred to be about 250 microns deep, for example, where it receives a substrate having a surface area of mm X 10 mm and a thickness of 200 microns.

It is further desired that there be formed a narrow groove 14 (FIG. 5A) near the recess 13. This narrow groove 14 so acts as to scrape off dirt deposited on the underside of the receptacle 15 by a source solution for the liquid phase epitaxial growth of semiconductor crystals.

The disc-shaped vessel 6 of said source solution is provided, as illustrated in FIGS. 6A and 6B, with four through holes each serving as a receptacle of said source solution. These four holes are arranged equidistantly from the central square aperture 6a so as to be superposed in turn on the recess 13 of the boat 7 when the vessel 6 is rotated. Further, the disc-shaped vessel 6 is provided on the underside with narrow grooves 16 (FIG. 6A), each of which is disposed between the adjacent ones of the receptacles 15. These narrow grooves 16 are effective like the aforementioned narrow groove 14 of the boat 7 to rub off dirt deposited on a semiconductor substrate due to accumulation of the source solution for liquid phase crystallization. In the receptacle 15 is received a prescribed source solution 17 for liquid phase crystallization. On the solution is placed, as shown in FIGS. 2 and 3, a weight 18 made of, for example, graphite. This weight 18 prevents the source solution 17 from making a poor contact with a semiconductor substrate 19 due to the meniscus of said solution 17, thus offering the advantage that even a small amount of source solution can attain the epitaxial growth of semiconductor crystals over a large area with the resultant saving of said solution.

In the cylindrical support 8, a pipe 21 containing a thermocouple 20 has its upper end portion inserted into the envelope 4 at its bottom. The uppermost end of said pipe 21 is fitted into a cavity 22 (FIG. 58) cut out at the bottom of the boat 7 to prevent it from jointly rotating with the vessel 6 when the latter turns. Said cavity 22 is formed at a point exactly facing the square recess 13 for receiving the semiconductor substrate 19 cut out in the opposite sides of the boat 7, thus enabling the temperature of said substrate 19 to be detected through the thermocouple 20.

According to the apparatus of this invention, the liquid phase epitaxial growth of semiconductor crystals is effected in an atmosphere of hydrogen or other inert gases. Said atmosphere is maintained by the gas which is introduced, as shown in FIG. 1, through a gas inlet 23 open to an interspace between the envelope 4 and cylindrical support 8, then through the cylindrical support 8, and finally through a gas outlet 24 open to the bottom of said cylindrical support 8 to be discharged to the outside.

There will now be described by reference to the appended drawings the method according to this invention of carrying out the liquid phase epitaxial growth of semiconductor crystals. For convenience of description, there is taken as an example an apparatus adapted to create said crystals in four layers by the LPE method. The apparatus is provided, as shown in FIG. 6A, with four source solution receptacles 15. The boat 7 has a recess 13 for receiving a semiconductor substrate 19, said recess being brought exactly under any of the receptacles 15 of the vessel 6 when it is rotated. Each receptacle 15 is filled with a source solution 17 required for the liquid phase epitaxial growth of semiconductor crystals on the substrate 19. Rotation of the vessel 6 causes the four source solution receptacles l5 successively to pass over the substrate 19 placed in the recess 13 of the boat 7, thereby carrying out the liquid phase epitaxial growth of semiconductor crystals in four layers.

The vessel 6 is rotated by the rotation drive shaft 9. In this case, the speed and process of rotation may be adjusted as need requires. That is, any process is permissible, if it attains the liquid phase epitaxial growth of semiconductor crystals by rotating the source solution vessel 6 relative to the semiconductor substrate 19, for example, by stopping the vessel 6 for a desired length of time at a point where the substrate 19 and source solution 7 face each other for the liquid phase epitaxial growth of semiconductor crystals, then shifting the succeeding source solution receptacle 15 by rotating the vessel 6 for a prescribed length of time and again stopping the vessel 6 for a prescribed length of time at a point where the substrate 19 and the source solution 7 of said succeeding receptacle 15 face each other for said epitaxial growth. It is also possible to rotate the boat 7 relative to the source solution vessel 6 with the latter kept at rest. The mechanical modification of the subject apparatus to attain this end will be apparent to those skilled in the art.

Where there is used a vessel 6 provided with more than four source solution receptacles 15, the vessel 6 and boat 7 need not have a much greater thickness in forming multiple layers of semiconductor cyrstals by the LPE method. In the case of the foregoing embodiment, for example, the overall thickness of the vessel 6 and boat 7 may be about 4 cm at most.

Since the vessel 6 is only required to have a slightly greater thickness, as described above, even in case there are formed more than four layers of semiconductor crystals, the vessel 6 can be better maintained at uniform temperature. Moreover, the receptacles 15 of the vessel 6 are symmetrically arranged relative to the center of the vessel 6, so that where there is used a cylindrical furnace 8 in a vertical position, the overall thickness of the solution 7 and substrate 19 only corresponds to an extremly small fraction of the vertical length of the furnace 8, ensuring the maintenance of uniform temperature. Accordingly, the liquid phase epitaxial growth of semiconductor crystals even in more than four layers can be realized using a cylindrical furnace whose manufacture has been made much easier. The foregoing description also applies where the semiconductor substrate 19 has a considerably large size.

Where the substrate 19 is desired to have a slightly higher temperature than the source solution 7 at the time of liquid phase formation of semiconductor crystals, then the object is attained by bringing the upper surface of the boat 7 to line A in the curve diagram of FIG. 7 showing a distribution of temperature in the cylindrical furnace 8. This operation is effected by means 26 (FIG. 1), for example, screws for adjusting the height of the furnace body 1 by moving it along its support pillar 25.

This invention is not limited to the aforementioned embodiment, but may also be applied in the case where the boat 7 is provided on its surface with more than four recesses for receiving substrates 19 and is rotated while the upper surfaces of the substrates 19 are made successively to contact the source solution 7 contained in a plurality of fixed receptacles 15. This process enables multiple LPE layers to be deposited simultaneously on many substrates, thereby elevating production efficiency.

The effect of this invention will be more fully appreciated by reference to the example which follows. For better understanding, there is taken the case where there is prepared from Ga, Al,As a heterojunction switching elementhaving a 4-layer structure of P-N-P- N. In this case there has only to be provided two symmetrically arranged source solution receptacles 15. Obviously, said receptacles need not be symmetrically disposed.

First, there is placed a substrate, for example, a mirror-polished GaAs single crystal, as shown in FIG. 5B, in the recess 13 of the boat 7. Then the vessel 6 and boat are fitted in turn to the drive shaft 9 from its bottom. In this case it is advised that the vessel 6 and boat 7 be so made to abut against each other as to prevent any of the source solution receptacles 15 from facing the substrate 19. Then the receptacles 15 are filled with a source solution for the liquid phase epitaxial growth of semiconductor crystals.

A first receptacle 15 is filled with Ga, GaAlAs and Zn as an acceptor impurity and a second receptacle 15 with Ga, GaAlAs and Te or Sn as a donor impurity. The assembly of the vessel 6 and boat 7 is introduced into the envelope 4 shown in FIG. 1. The boat 7 is fixed to the upper end of the cylindrical support 8 disposed in the envelope 4. The drive shaft 9 is connected to a rotation control system (not shown). On the source solution 17 is placed the weight 18 to cover it, as well as to prevent the substrate from making a poor contact with the source solution due to the occurrence of its meniscus. Hydrogen gas is introduced into the envelope 4 through the gas inlet 23 to maintain an atmosphere of hydrogen therein.

The temperature of the subject apparatus heated by the heater 3 is controlled in the following manner. The solubility of GaAlAs in Ga is determined from a curve indicating the solubility of As in Ga. Initially, heating is continued at a temperature of 850 C until As attains a supersaturating condition with respect to Ga. After GaAlAs is fully dissolved in Ga, the furnace temperature is reduced at the rate of 1 C per minute by means of the heater 3. Where said temperature falls about 5 C, the vessel 6 is rotated to contact the solution of the first receptacle 15 with the substrate 19. At this time the crystals of GaAlAs are grown at the rate of 0.4 ,u/ C, so that the temperature is further reduced about 5 C to provide an N type LPE layer of GaAlAs about 2 microns thick. During this time, the vessel 6 is kept at rest. The substrate 19 continues to contact the solution of the first receptacle 15 to form a P type LPE layer of GaAlAs about 2 microns thick. The vessel 6 is intermittently rotated while the above-mentioned operation is repeated to prepare a multilayer semiconductor element of N (substrate) -N-P-N-P structure.

Impurities Zn and Te or Sn are mixed according to the properties demanded of the resultant switching element. The temperature of the cylindrical furnace at which the aforesaid multilayer semiconductor element is formed may generally be represented by the curve of FIG. 8, which shows the relationship between the period during which the substrate contacts the source solution and the resultant temperature drop. Namely, temperature is permitted to fall AT, C during the initial period of t minutes to form a prescribed LPE layer of Ga Al As on the substrate by its contact with the source solution of a given receptacle. When the substrate contacts the solution of the succeeding receptacle, temperature is made further to decrease AT C during a period of t, minutes for creation of another prescribed LPE layer of Ga Al,As. Hereafter, the temperature of the cylindrical furnace is successively let to fall as AT and AT during an interval of t and t minutes respectively to deposit other prescribed LPE layers of Ga, ,,Al,,As on the substrate.

What we claim is:

1. A method for the liquid phase epitaxial growth of semiconductor crystals upon a substrate in multiple layers which comprises:

providing a plurality of source solutions for epitaxial growth of semiconductor crystals confined in receptacles arranged equidistantly about a central axis, providing a substrate that presents a flat surface upon which semiconductor crystals are to be grown,

positioning said substrate about said central axis so that said flat surface may contact one of said source solutions,

epitaxially growing a layer of semiconductor crystal upon said flat surface from said one source solution,

bringing another of said source solutions into contact with said crystal layer upon said substrate by relative rotation of the source solutions and said substrate about said central axis, and

epitaxially growing a second layer of semiconductor crystal upon said substrate from said another source solution.

2. A method for the liquid phase epitaxial growth of semiconductor crystals which comprises providing a disc-shaped vessel having a plurality of receptacles containing a source solution for said epitaxial growth equidistantly arranged from the center of the vessel; providing a similarly disc-shaped boat disposed close to the underside of said vessel and provided with a recess containing a substrate on which there is to be formed semiconductor crystals by liquid phase epitaxial growth, said recess being cut out in that side of the boat which faces the vessel so as to be brought exactly under any of the receptacles of the vessel when it is rotated; positioning said substrate under one of said receptacles with the top surface of the substrate in contact with source solution contained in the receptacle, epitaxially growing a layer of semiconductor crystal upon said surface from source solution in contact therewith; rotating said vessel to position said substrate under another of said receptacles; and epitaxially growing a second layer of semiconductor crystal upon said substrate from source solution contained in said another receptacle.

6. The method according to claim 2 wherein the source solution vessel has a graphite weight fitted into each receptacle so as to connect the source solution contained therein, thereby preventing the substrate from making a poor contact with said solution due to the possible occurrence of its meniscus.

7. The method according to claim 2 wherein the boat is provided on the underside with a cavity into which there is inserted the uppermost end of a pipe containing a thermocouple. 

2. A method for the liquid phase epitaxial growth of semiconductor crystals which comprises providing a disc-shaped vessel having a plurality of receptacles containing a source solution for said epitaxial growth equidistantly arranged from the center of the vessel; providing a similarly disc-shaped boat disposed close to the underside of said vessel and provided with a recess containing a substrate on which there is to be formed semiconductor crystals by liquid phase epitaxial growth, said recess being cut out in that side of the boat which faces the vessel so as to be brought exactly under any of the receptacles of the vessel when it is rotated; positioning said substrate under one of said receptacles with the top surface of the substrate in contact with source solution contained in the receptacle, epitaxially growing a layer of semiconductor crystal upon said surface from source solution in contact therewith; rotating said vessel to position said substrate under another of said receptacles; and epitaxially growing a second layer of semiconductor crystal upon said substrate from source solution contained in said another receptacle.
 3. The method according to claim 2 wherein the boat is provided with a narrow groove in that side which faces the source solution vessel.
 4. The method according to claim 2 wherein the source solution vessel is provided with a plurality of narrow grooves in that side which faces the boat, each of said grooves being disposed between the adjacent ones of the receptacles.
 5. The method according to claim 4 wherein the boat is provided with a narrow groove in that side which faces the source solution vessel.
 6. The method according to claim 2 wherein the source solution vessel has a graphite weight fitted into each receptacle so as to connect the source solution contained therein, thereby preventing the substrate from making a poor contact with said solution due to the possible occurrence of its meniscus.
 7. The method according to claim 2 wherein the boat is provided on the underside with a cavity into which there is inserted the uppermost end of a pipe containing a thermocouple. 